Sensitive technique for metal-void detection

Abstract

A categorization of a particular semiconductor wafer based on void size is obtained from sigma data and T0.1% failure data that has been obtained from wafers subjected to isothermal testing. The sigma data and the T0.1% failure data for the particular wafer is compared to stored data corresponding to ranges for sigma and T0.1% data for each of a plurality of void categories, and the particular wafer is categorized based on the stored data. The T0.1% failure data is computed based on a T50% failure data and the sigma value, so that small sample sizes can be utilized to obtain the stored data.

What is claimed is: 
     
       1. A method for determining a void size of a semiconductor wafer, the method comprising the steps of: a) obtaining test data for a plurality of semiconductor wafers, the test data corresponding to time-to-fail for the semiconductor wafers;   b) categorizing the plurality of semiconductor wafers into a plurality of categories based on sizes of voids formed in the semiconductor wafers;   c) from the test data obtained in the step a), determining a time when a certain percentage of the wafers have failed;   d) from the test data obtained in the step a), determining a sigma value for the test data;   e) from the time obtained in the step c) and the sigma value obtained in the step d), deriving a time when x percent of the wafers have failed, x being a number less than or equal to one; and   f) categorizing the semiconductor wafer based on the sigma value obtained in the step d) and the time derived in the step e).   
     
     
       2. The method according to claim 1, wherein the step b) is performed by cutting each of the wafers to obtain a cross-sectional view of each of the wafers, and placing the cut wafers under a microscope to determine sizes of the voids formed therein. 
     
     
       3. The method according to claim 1, wherein the certain percentage is 50%. 
     
     
       4. The method according to claim 3, wherein x is 0.1. 
     
     
       5. The method according to claim 3, wherein x is 0.01. 
     
     
       6. The method according to claim 3, wherein x is 1.0. 
     
     
       7. The method according to claim 1, wherein the wafer is categorized into at least an acceptable void size and an unacceptable void size. 
     
     
       8. The method according to claim 1, wherein the wafer is categorized in the step f) without cutting the wafer, and wherein a visual inspection of the wafer is not performed during the categorization of the wafer in the step f).   
     
     
       9. A method for categorizing a particular semiconductor wafer based on void size, the method comprising the steps of: a) subjecting a plurality of semiconductor wafers to isothermal testing, the plurality of semiconductor wafers not including the particular semiconductor wafer;   b) obtaining time-to-fail data and sigma data for the plurality of semiconductor wafers subjected to the isothermal testing, the time-to-fail data being obtained for at least a certain percentage of the wafers having failed;   c) from the time-to-fail data and the sigma data obtained in the step b), computing a time when x percent of the wafers have failed, x being a number less than or equal to one;   d) examining each of the plurality of semiconductor wafers under a microscope and categorizing each of the plurality of wafers based on sizes of voids formed in each of the plurality of semiconductor wafers;   e) determining a range of values for the time-to-fail data and the sigma data for each of the categories in the step d);   f) subjecting the particular semiconductor wafer to isothermal testing;   g) obtaining time-to-fail data and sigma data for the particular semiconductor wafer subjected to the isothermal testing; and   h) categorizing the particular semiconductor wafer into a catergory based on the sigma data and the time-to-fail data obtained in the step g), and the range of values for the time-to-fail data and the sigma data obtained in the step e).   
     
     
       10. The method according to claim 9, wherein the step d) is performed by cutting each of the plurality of wafers to obtain a cross-sectional view of each of the plurality of wafers, and placing the cut wafers under a microscope to determine sizes of the voids formed therein. 
     
     
       11. The method according to claim 9, wherein the certain percentage is 50%. 
     
     
       12. The method according to claim 11, wherein x is 0.1. 
     
     
       13. The method according to claim 11, wherein x is 0.01. 
     
     
       14. The method according to claim 11, wherein x is 1.0. 
     
     
       15. The method according to claim 9, wherein the particular wafer is categorized without cutting the particular wafer, and wherein a visual inspection of the particular wafer is not performed during the categorization of the particular wafer.   
     
     
       16. The method according to claim 9, further comprising the step of determining the value x used in the step c) based on a degree of conservatism in the categorization of the particular wafer. 
     
     
       17. The method according to claim 1, wherein the particular wafer is categorized into at least an acceptable void size and an unacceptable void size. 
     
     
       18. A method of categorizing a wafer based on void size, the method comprising the steps of: a) subjecting the wafer to isothermal testing, the wafer being subjected to the isothermal testing along with a set of other wafers;   b) obtaining time-to-fail data and sigma data for the wafer, based on data obtained from the isothermal testing in the step a);   c) comparing the time-to-fail data and the sigma data for the wafer against stored ranges of time-to-fail data and stored ranges of sigma data for a plurality of void categories; and   d) determining one of the void categories for the wafer based on the comparing performed in the step c).